Photosensitive element having reinforcing particles and method for preparing a printing form from the element

ABSTRACT

The invention provides a photosensitive element and a method for preparing a printing form from the element. The photosensitive element includes a layer of a photosensitive composition containing a binder, a monomer, and a Norrish type II photoinitiator, wherein the photosensitive layer has a transmittance to actinic radiation of less than 20% and contains reinforcing particles of graphene and/or carbon nanotubes.

BACKGROUND OF THE INVENTION

1. Field of the Disclosure

This invention pertains to a photosensitive element and a method forpreparing a printing form from the element, and in particular to aphotosensitive element having a photopolymerizable compositioncontaining an additive and a method for treating the element to form asurface suitable for printing.

2. Description of Related Art

Flexographic printing plates are well known for use in printing surfaceswhich range from soft and easy to deform to relatively hard, such aspackaging materials, e.g., cardboard, plastic films, aluminum foils,etc. Flexographic printing plates can be prepared from photosensitiveelements containing photopolymerizable compositions, such as thosedescribed in U.S. Pat. Nos. 4,323,637 and 4,427,759. Thephotopolymerizable compositions generally comprise an elastomericbinder, at least one monomer and a photoinitiator. Photosensitiveelements generally have a photopolymerizable elastomeric layerinterposed between a support and a coversheet or multilayer coverelement. Upon imagewise exposure to actinic radiation,photopolymerization of the photopolymerizable layer occurs in theexposed areas, thereby curing and rendering insoluble the exposed areasof the layer. The element is treated with a suitable solution, e.g.,solvent or aqueous-based washout, or thermally, to remove the unexposedareas of the photopolymerizable layer leaving a printing relief whichcan be used for flexographic printing.

In some instances it may be desirable to directly engrave the printingform with laser radiation of sufficient intensity to ablate theelastomeric material and form a surface, e.g., relief, suitable forprinting. Photopolymerizable printing elements are often photochemicallyreinforced by overall exposing the element to actinic radiation beforeimagewise ablating with laser radiation. Particulate material that issensitive to the laser radiation may be added to the photosensitivecomposition in order to enhance the engraving efficiency of the elementand to reinforce the mechanical properties of the element.

U.S. Pat. Nos. 5,798,202 and 5,804,353 disclose process for make aflexographic printing plate by laser engraving one or multiplyreinforced elastomeric layers on a flexible support. The processinvolves reinforcing and laser engraving the one or multiple reinforcedelastomeric layers. The elastomeric layer can be reinforcedmechanically, or photochemically, or thermochemically, or combinationsthereof. Mechanical reinforcement is provided by incorporatingreinforcing agents, such as finely divided particulate material, intothe elastomeric layer. Carbon black and graphite are cited as suitablereinforcing agents. Photochemical reinforcement is accomplished byincorporating photohardenable materials into the elastomeric layer andexposing the layer to actinic radiation. Photohardenable materialsinclude photocrosslinkable and photopolymerizable systems having aphotoinitiator or photoinitiator system. Both patents provide examplesof photosensitive elements that are only mechanically reinforced withcarbon black, in which the amount of carbon black in the elastomericlayer was from 1 to 25% by weight of the elastomeric layer. Both patentsalso provide examples of photosensitive elements that are bothreinforced mechanically with carbon black in the elastomeric layer andreinforced photochemically. However, in U.S. Pat. No. 5,798,202 theamount of the reinforcing agent in the elastomeric layer wasconsiderably less (compared to elastomeric layers that were onlymechanically reinforced) in order to photochemically reinforce theelastomeric layer which had 2-phenyl-2,2-dimethoxy acetophenone (whichis a derivative of acetophenone) as a photoinitiator. The amount ofcarbon black in the photosensitive elements of the examples where theelastomeric layer was mechanically and photochemically reinforced wasless than 0.23% by weight of the elastomeric layer, so that the actinicradiation could adequately penetrate the layer and photochemicallyreinforce the element. An example of U.S. Pat. No. 5,804,353 prepared amultilayer plate in which a top layer of a photosensitive compositionincluded carbon black as mechanical reinforcing particles at 18.7% byweight, and 2-isopropylthioxanthone and ethyl-p-dimethylaminobenzoate asa photoinitiator system. However, the multilayer plate was laserengraved for use as a flexographic printing plate.

Flexographic printing is a form of relief printing in which the printingform prints from an image area, where the image area of the printingform is raised and the non-image area is depressed. Gravure printing isa method of printing in which the printing form prints from an imagearea, where the image area is depressed and consists of small recessedcups or wells to contain the ink or printing material, and the non-imagearea is the surface of the form. However, gravure printing forms areexpensive and require considerable time and material to produce. Agravure cylinder is essentially made by electroplating a copper orchrome layer onto a base roller, and then engraving the image composedof the small recessed cells or wells digitally by a diamond tipped orlaser etching machine.

It is contemplated that the photopolymerizable relief printing formcould be used for gravure-like printing. Gravure-like printing issimilar to gravure printing except that a relief printing form is usedwherein the image area is depressed and consists of recesses areasforming wells to carry the ink which transfer during printing. However,the mechanical properties, such as abrasion resistance, tensilestrength, and stiffness of the photopolymerizable relief printing formwould need to be enhanced in order for the element to effectivelyfunction as or similar to a conventional gravure cylinder. It isexpected that the recessed areas of the elastomeric flexographicprinting form that carry the ink could change volume during ink transferor nip contact with the substrate rendering unacceptable image quality.Also, conventional gravure printing typically uses a doctor blade toregulate the amount of ink transferred to the recessed areas of agravure cylinder. But the elastomeric flexographic printing form ingravure-like application is not likely sufficiently resistant to theabrasion and wear associated with continuous contact of the doctorblade. Hereto, particulate material may be added to the photosensitivecomposition in order to reinforce the mechanical properties of theelement. A photopolymerizable printing form functioning for gravure-likeprinting could be easily imaged by laser engraving or by conventionalimagewise exposure and treatment as described previously.

Additionally, the solvent resistance of the photopolymerizable reliefprinting form also needs to be enhanced in order for the printing formto effectively function as or similar to a conventional gravurecylinder. Most inks used in conventional gravure printing typicallycontain toluene, which is a potent solvent. An elastomeric flexographicprinting form is not sufficiently resistant to toluene or other strongsolvents, and may dissolve or swell upon contact with the ink,particularly over time. Swelling of the printing form on press willresult in poor quality of the printed images. It is also possible thatthe solvent ink may cause some of the components from the printing forto leach out, which could cause complete failure of the printing form onpress.

A problem associated with the photosensitive printing forms having anelastomeric layer which is reinforced both mechanically andphotochemically is that laser engraving does not efficiently remove theelastomeric material to provide desired relief quality, and ultimatelyprinting quality. Photochemically reinforcing the element can providethe desired properties for engraving as well as in its end-use as aprinting plate. However, the presence of a particulate additive, like areinforcing agent, tends to reduce the penetration of the ultravioletradiation required to photochemically reinforce the element. If theelastomeric layer is insufficiently cured during photochemicalreinforcement, the laser radiation cannot effectively remove thematerial and poor relief quality of the engraved area results. Further,the debris resulting from laser engraving tends to be tacky and isdifficult to completely remove from the element. Additionally, if theelement is not sufficiently photochemically reinforced the requiredend-use properties as a flexographic or gravure-like printing plate arenot achieved. These problems tend to be exacerbated with increasingconcentration of the additive that enhances engraving efficacy or themechanical properties for gravure-like printing.

SUMMARY OF THE INVENTION

The present invention provides a photosensitive element for use as aprinting form comprising a layer of a photosensitive compositioncomprising a binder, a monomer, and a Norrish type II photoinitiator,wherein the photosensitive layer contains reinforcing particles thatprovide the layer with a transmittance to actinic radiation of less than20%, and wherein the reinforcing particles are selected from the groupconsisting of graphene, carbon nanotubes, and combinations thereof.

In accordance with another aspect of this invention there is provided amethod for preparing a printing form from a photosensitive elementcomprising a layer of a photosensitive composition comprising a binder,a monomer, and a Norrish type II photoinitiator, wherein thephotosensitive layer contains reinforcing particles that provide thelayer with a transmittance to actinic radiation of less than 20%, andwherein the reinforcing particles are selected from the group consistingof graphene, carbon nanotubes, and combinations thereof. The methodincludes exposing the photosensitive element to actinic radiation; andtreating the exposed photosensitive element to form a printing formhaving a surface suitable for printing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention provides a photosensitive element and a method ofpreparing a printing form from the photosensitive element. Thephotosensitive element can be used as a printing form for reliefprinting, e.g., flexographic and letterpress, from raised surfaces ofthe relief; or as a gravure-like printing form for gravure printing fromrecessed surfaces (or wells) of the relief. The photosensitive elementis a photopolymerizable element formed of a layer of a photosensitivecomposition that includes a binder, at least one monomer, aphotoinitiator, and reinforcing particles. In particular, thephotosensitive element has at least one photosensitive layer thatincludes with the binder and the at least one monomer, reinforcingparticles that provide the layer with a transmittance to actinicradiation of less than about 20%, in some instances less than 15%, andin other instances less than 10%, and the photoinitiator which is aNorrish type II photoinitiator.

The photosensitive printing element of the present invention has severaladvantages. The presence of the reinforcing particles in combinationwith the Norrish type II photoinitiator in the photopolymerizable layerprovides improvement in the mechanical properties of the photosensitiveelement for use in both relief printing and gravure-like printingapplications. The photosensitive printing element can withstand therigorous conditions of associated with treatment to form a reliefsurface suitable for high quality printing of fine features such aslines and highlight dots of the resulting printing form. Also thephotosensitive printing element can resist wear of the printing reliefsurface and dot chipping of relief elements in the relief surface. Thepresence of the reinforcing agent in the photopolymerizable layer canimprove the productivity of a method to prepare a printing form from thephotosensitive element by enhancing the efficacy of engraving, orremoval in depth, of cured material from the photopolymerizable layer.The presence of the reinforcing particles in the photopolymerizablelayer can also provide an improvement in the solvent resistance of thephotosensitive element for use in both relief printing and gravure-likeprinting applications. Surprisingly, the solvent resistance of thephotosensitive element is markedly increased by a photopolymerizablelayer containing reinforcing agents selected from the group consistingof graphene, carbon nanotubes, and combinations thereof. The presence ofthe Norrish type II photoinitiator permits effective initiation andpenetration of the actinic radiation, e.g. ultraviolet radiation,required to photochemically reinforce the element despite the presenceof the reinforcing particles that reduce the transmission of theradiation to less than less than about 20%, in some instances less than15%, and in other instances less than 10%. The Norrish type IIphotoinitiator with a co-initiator is particularly effective atinitiating the photopolymerization of the present photosensitiveelement. The use of a ketosulphone compound as the Norrish type IIphotoinitiator is a particularly effective initiator for the presentphotosensitive element.

The photosensitive printing element includes at least one layer of aphotosensitive composition that can be considered a photopolymerizablecomposition. As used herein, the term “photopolymerizable” is intendedto encompass systems that are photopolymerizable, photocrosslinkable, orboth. In cases where the composition layer comprises more than onephotopolymerizable layer on the substrate, the composition of each ofthe photopolymerizable layers can be the same or different from any ofthe other photopolymerizable layers. The photopolymerizable layer is asolid layer composed of the binder, a monomer, a photoinitiator, andreinforcing particles. In one embodiment, the layer is elastomeric. Thephotoinitiator has sensitivity to actinic radiation. Throughout thisspecification actinic radiation will include ultraviolet radiationand/or visible light. In one embodiment, the solid layer of thephotopolymerizable composition can be imagewise exposed and treated withone or more solutions and/or heat to form a relief suitable forprinting. In another embodiment, the solid layer of thephotopolymerizable composition is reinforced photochemically by overallexposure to actinic radiation and mechanically reinforced by thereinforcing agents, and then engraved by exposure to laser radiation toselectively remove material to form a surface suitable for printing. Asused herein, the term “solid” refers to the physical state of the layerthat has a definite volume and shape and resists forces that tend toalter its volume or shape. The layer of the photopolymerizablecomposition is solid at room temperature, which is a temperature betweenabout 5° C. and about 30° C. A solid layer of the photopolymerizablecomposition may be polymerized (photohardened), or unpolymerized, orboth.

Unless otherwise indicated, the terms “photosensitive element” and“printing form” encompass elements or structures in any form suitablefor printing, including, but not limited to, flat sheets, plates,seamless continuous forms, cylindrical forms, plates-on-sleeves, andplates-on-carriers. It is contemplated that printing form resulting fromthe photosensitive element has end-use printing applications for reliefprinting, such as flexographic and letterpress printing, and forgravure-like printing. Various processes may be used to produce aprinting form from the photosensitive element and create a reliefstructure suitable for relief or gravure-like printing.

The photosensitive composition forming the layer is composed of abinder, a monomer, a photoinitiator, and reinforcing particles. Thebinder may be photoactive itself or may act as a matrix for one or moreof the photoactive components, i.e., the monomer and the photoinitiator.The binder is a dispersible polymeric component that imparts desiredphysical and chemical characteristics to the exposed and unexposedphotosensitive composition. The polymer is typically, but notnecessarily, preformed. The polymer is not limited and includes polymersthat are linear, branched, radial, comb, and can become interpenetratingnetworks.

The binder can be a preformed polymer that serves as a matrix for themonomer and photoinitiator system prior to exposure and is a contributorto the physical properties of the photopolymer both before and afterexposure. Addition of the binder can allow the imaging element to bemanufactured and handled as a dry film. As is the case withphotoinitiators and monomers, the selection criteria for binders varywith the application. Molecular weight, glass transition temperature,flexibility, chemical resistance, solubility, toughness, and tensilestrength, as well as cost and availability are among the factors thatgovern binder selection. The binder should be of sufficient molecularweight and have sufficiently high glass transition temperature that afilm is formed when the composition is coated. Suitable binders can havewidely varying molecular weights, from as low as 25,000 to greater than300,000, to as much as 1,200,000 have been described. Unless otherwiseindicated the molecular weight of the polymeric binder is a meanmolecular weight Mw determined with the aid of gel permeationchromatography using polystyrene standards.

The binder can be a single polymer or mixture of polymers. The bindercan include a combination of materials, which under appropriatepreparation or end-use conditions, provide the resulting element withcharacteristics suitable for use as a printing form. The binder can bethermoplastic, elastomeric thermoplastic, elastomeric, ornon-elastomeric. When a layer containing a thermoplastic elastomericbinder is reinforced photochemically, the layer remains elastomeric butis no longer thermoplastic after such reinforcement. Any of thefollowing binders can be used alone or in combination to provide thephotosensitive element with desired adhesion, flexibility, hardness,oxygen permeability, moisture sensitivity and other mechanical orchemical properties required for its processing or end use. Othercomponents in the composition should be compatible with the binder tothe extent that a clear, non-cloudy photosensitive layer is produced.The binder is present in the photosensitive composition from 10 to 95%,preferably 40-80%, and most preferably 45-65%, by weight, based on theweight of the composition.

Polymers suitable for use as the binder include elastomeric polymers ofnatural or synthetic polymers of conjugated diolefin hydrocarbons, suchas, isoprene or butadiene. Examples of suitable elastomeric bindersinclude, but is not limited to, natural rubber, polybutadiene,polyisoprene, copolymers of styrene and butadiene, nitrile-butadienerubber, acrylonitrile-butadiene rubber, acrylonitrile-butadiene-styrenecopolymers, methacrylate-acrylonitrile-butadiene-styrene copolymers,butyl rubber, copolymers of styrene and isoprene, polynorbonene rubber,and ethylene-propylene-diene rubber (EPDM). Also suitable as the binderare ethylene-propylene, ethylene-acrylate ethylene-vinyl acetate oracrylate rubbers. Block copolymers of vinyl-substituted aromatichydrocarbons and conjugated dienes are also suitable for use as thebinder. Further examples of elastomeric binders include,polyalkyleneoxides; polyphosphazenes; elastomeric polymers andcopolymers of acrylates and methacrylate; elastomeric polyurethanes andpolyesters; elastomeric polymers and copolymers of olefins such asethylene-propylene copolymers and non-crosslinked EPDM; elastomericcopolymers of vinyl acetate and its partially hydrogenated derivatives.The polymers may be used alone or in combination with the AB blockcopolymers described herein. Additional examples of elastomericmaterials suitable for use as the binder are described in PlasticTechnology Handbook, Chandler et al., Ed., (1987).

Polymers that are curable by means other than photochemical reaction,such as thermally curable and room temperature curable silicone rubbers,can be included to the extent that these polymers can be effectivelycombined with the primary photochemically curable system.

Still further examples of other suitable binders alone or in combinationinclude: polyacrylate and alpha-alkyl polyacrylate esters, e.g.,polymethyl methacrylate and polyethyl methacrylate; polyvinyl esters,e.g., polyvinyl acetate, polyvinyl acetate/acrylate, polyvinylacetate/methacrylate and hydrolyzed polyvinyl acetate; ethylene/vinylacetate copolymers; polystyrene polymers and copolymers, e.g., withmaleic anhydride and esters; vinylidene chloride copolymers, e.g.,vinylidene chloride/acrylonitrile; vinylidene chloride/methacrylate andvinylidene chloride/vinyl acetate copolymers; polyvinyl chloride andcopolymers, e.g., poly(vinyl chloride/vinyl acetate); polyvinylpyrrolidone and copolymers, e.g., poly(vinyl pyrrolidone/vinyl acetate)saturated and unsaturated polyurethanes; high molecular weightpolyethylene oxides of polyglycols having average molecular weights fromabout 4,000 to 1,000,000; epoxides, copolyesters, e.g., those preparedfrom the reaction product of a polymethylene glycol of the formulaHO(CH₂)_(n)OH, where n is a whole number 2 to 10 inclusive, and (1)hexahydroterephthalic, sebacic and terephthalic acids, (2) terephthalic,isophthalic and sebacic acids, (3) terephthalic and sebacic acids, (4)terephthalic and isophthalic acids, and (5) mixtures of copolyestersprepared from said glycols and (i) terephthalic, isophthalic and sebacicacids and (ii) terephthalic, isophthalic, sebacic and adipic acid;nylons or polyamides, e.g., N-methoxymethyl polyhexamethylene adipamide;polyvinyl alcohols; cellulose and cellulose derivatives, such as,cellulose esters, e.g., cellulose acetate, cellulose acetate succinateand cellulose acetate butyrate, and cellulose ethers, e.g., methylcellulose, ethyl cellulose and benzyl cellulose; polycarbonates;polyvinyl acetal, e.g., polyvinyl butyral, polyvinyl formal; andpolyformaldehydes.

The binder can be soluble, swellable, or dispersible in aqueous,semi-aqueous, water, or organic solvent washout solutions. Binders whichcan be removed by treating in aqueous or semi-aqueous developers havebeen disclosed by Proskow, in U.S. Pat. No. 4,177,074; Proskow in U.S.Pat. No. 4,431,723; Worns in U.S. Pat. No. 4,517,279; Suzuki et al. inU.S. Pat. No. 5,679,485 and U.S. Pat. No. 5,830,621; and Sakurai et al.in U.S. Pat. No. 5,863,704. The block copolymers discussed in Chen, U.S.Pat. No. 4,323,636; Heinz et al., U.S. Pat. No. 4,430,417; and Toda etal., U.S. Pat. No. 4,045,231 can be treated by wash out in organicsolvent solutions. Generally, the binders that are suitable for washoutdevelopment are also suitable for thermal treatment wherein theunpolymerized areas of the photopolymerizable layer soften, melt, orflow upon heating. In the case where aqueous development of thephotosensitive composition is desirable, the branched polymer productand/or the binder should contain sufficient acidic or other groups torender the composition processible in aqueous developer. Usefulaqueous-processible binders include those disclosed in U.S. Pat. No.3,458,311; U.S. Pat. No. 4,273,857; U.S. Pat. No. 6,210,854; U.S. Pat.No. 5,679,485; U.S. Pat. No. 6,025,098; U.S. Pat. No. 5,830,621; U.S.Pat. No. 5,863,704; and U.S. Pat. No. 5,889,116. Useful amphotericpolymers include interpolymers derived from N-alkylacryl-amides ormethacrylamides, acidic film-forming comonomer and an alkyl orhydroxyalkyl acrylate such as those disclosed in U.S. Pat. No.4,293,635.

In one embodiment, the binder is a thermoplastic elastomeric blockcopolymer composed of non-elastomeric blocks and elastomeric blocks,where A represents a non-elastomeric block of a vinyl-substitutedaromatic hydrocarbons, and B represents an elastomeric block ofconjugated dienes. Styrene and methylstyrene are examples of suitablevinyl-substituted aromatic hydrocarbons. Polybutadiene and polyisopreneare examples of conjugated dienes, which may be 1,2- or 1,4-linked. Theblock copolymers can be linear block copolymers, radial blockcopolymers, star block-copolymers, or quasi-radial block copolymers. Theblock copolymer may be fully or partially hydrogenated. The binder ofthe block copolymer can include tri-block A-B-A copolymers, but can alsobe two block copolymers of the A-B type, or those comprising a pluralityof alternating elastomeric and thermoplastic blocks, for exampleA-B-A-B-A. Block copolymers are described in U.S. Pat. Nos. 4,323,636,4,430,417, and 4,045,231. Particularly suitable thermoplasticelastomeric tri-block copolymers include poly(styrene/isoprene/styrene)(SIS) block copolymers, poly(styrene/butadiene/styrene) (SBS) blockcopolymers, and combinations of SIS and SBS block copolymers. It is alsopossible to employ mixtures of two or more different block copolymers.For example, ABA block copolymers can contain AB diblock copolymer. Alsoincluded as block copolymers such as, for example,polystyrene-poly(ethylenebutylene)-polystyrene (SEBS) block copolymer,and polystyrene-poly(ethylene-ethylene propylene)-polystyrene (SEEPS)block copolymer. The non-elastomer to elastomer ratio is preferably inthe range of from 10:90 to 35:65. The block copolymer as binder may bepresent in an amount of 40% to 95% by weight of the photosensitivelayer.

In some embodiments, it may be desirable for the binder to includependant alkyl groups of 1 to 6 carbon atoms, such as for example,methyl, ethyl, and tert-butyl, sufficient for donating hydrogen uponreaction with the Norrish type II photoinitiator. For example, ABA blockcopolymers may include vinyl group/s that are pendant) in the mid-block,i.e., conjugated diene, which are available for hydrogen donation andcan react with the Norrish type II photoinitiator.

One embodiment of a suitable multi-component system includes anelastomeric polyurethane composition. An example of a suitablepolyurethane elastomer is the reaction product of (i) an organicdiisocyanate, (ii) at least one chain extending agent having at leasttwo free hydrogen groups capable of polymerizing with isocyanate groupsand having at least one ethylenically unsaturated addition polymerizablegroup per molecule, and (iii) an organic polyol with a minimum molecularweight of 500 and at least two free hydrogen containing groups capableof polymerizing with isocyanate groups. For a description of some ofthese materials see U.S. Pat. Nos. 5,015,556 and 5,175,072.

In another embodiment, the binder encompasses core shell microgels andblends of microgels and preformed macromolecular polymers, such as thosedisclosed in Fryd et al., U.S. Pat. No. 4,956,252 and Quinn et al., U.S.Pat. No. 5,707,773.

Monomers that can be used in the composition activated by actinicradiation are well known in the art, and include, but are not limitedto, addition-polymerization ethylenically unsaturated compounds havingrelatively low molecular weights that is, molecular weights generallyless than about 30,000, and preferably less than about 5000. Unlessdescribed otherwise in the specification, the molecular weight is theweighted average molecular weight. The addition polymerization compoundmay also be an oligomer, and can be a single or a mixture of oligomers.If a polyacrylol oligomer is used, the oligomer should preferably have amolecular weight greater than 1000. The composition can contain a singlemonomer or a combination of monomers. The monomer compound capable ofaddition polymerization is present in at least an amount of 5%,preferably 10 to 20%, by weight of the composition.

Suitable monomers include, but are not limited to, acrylate monoestersof alcohols and polyols; acrylate polyesters of alcohols and polyols;methacrylate monoesters of alcohols and polyols; and methacrylatepolyesters of alcohols and polyols; where the alcohols and the polyolssuitable include alkanols, alkylene glycols, trimethylol propane,ethoxylated trimethylol propane, pentaerythritol, and polyacrylololigomers. Other suitable monomers include acrylate derivatives andmethacrylate derivatives of isocyanates, esters, epoxides, and the like.A combination of monofunctional and multifunctional acrylates ormethacrylates may be used.

Examples of suitable monomers include the following: t-butyl acrylate,hexanediol diacrylate, hexanediol dimethyacrylate, 1,5-pentanedioldiacrylate, N,N-diethylaminoethyl acrylate, ethylene glycol diacrylate,1,4-butanediol diacrylate, diethylene glycol diacrylate, hexamethyleneglycol diacrylate, 1,3-propanediol diacrylate, decamethylene glycoldiacrylate, decamethylene glycol dimethacrylate, 1,4-cyclohexanedioldiacrylate, 2,2-dimethylolpropane diacrylate, glycerol diacrylate,tripropylene glycol diacrylate, glycerol triacrylate, trimethylolpropanetriacrylate, pentaerythritol triacrylate, polyoxyethylatedtrimethylolpropane triacrylate and trimethacrylate and similar compoundsas disclosed in U.S. Pat. No. 3,380,831, 2,2-di(p-hydroxyphenyl)-propanediacrylate, pentaerythritol tetraacrylate,2,2-di-(p-hydroxyphenyl)-propane dimethacrylate, triethylene glycoldiacrylate, polyoxyethyl-2,2-di-(p-hydroxyphenyl)-propanedimethacrylate, di-(3-methacryloxy-2-hydroxypropyl)ether of bisphenol-A,di-(2-methacryloxyethyl) ether of bisphenol-A,di-(3-acryloxy-2-hydroxypropyl) ether of bisphenol-A,di-(2-acryloxyethyl) ether of bisphenol-A,di-(3-methacryloxy-2-hydroxypropyl) ether of tetrachloro-bisphenol-A,di-(2-methacryloxyethyl) ether of tetrachloro-bisphenol-A,di-(3-methacryloxy-2-hydroxypropyl) ether of tetrabromo-bisphenol-A,di-(2-methacryloxyethyl) ether of tetrabromo-bisphenol-A,di-(3-methacryloxy-2-hydroxypropyl) ether of 1,4-butanediol,di-(3-methacryloxy-2-hydroxypropyl) ether of diphenolic acid,triethylene glycol dimethacrylate, polyoxypropyl one trimethylol propanetriacrylate (462), ethylene glycol dimethacrylate, butylene glycoldimethacrylate, 1,3-propanediol dimethacrylate, 1,2,4-butanetrioltrimethacrylate, 2,2,4-trimethyl-1,3-pentanediol dimethacrylate,pentaerythritol trimethacrylate, I-phenyl ethylene-1,2-dimethacrylate,pentaerythritol tetramethacrylate, trimethylol propane trimethacrylate,1,5-pentanediol dimethacrylate, diallyl fumarate, 1,4-benzenedioldimethacrylate, 1,4-diisopropenyl benzene, and 1,3,5-triisopropenylbenzene.

Other examples of suitable monomers include acrylate and methacrylatederivatives of isocyanates, esters, epoxides and the like. In someend-use printing forms it may be desirable to use monomer that provideelastomeric properties to the element. Examples of elastomeric monomersinclude, but are not limited to, acrylated liquid polyisoprenes,acrylated liquid butadienes, liquid polyisoprenes with high vinylcontent, and liquid polybutadienes with high vinyl content, (that is,content of 1-2 vinyl groups is greater than about 20% by weight).

Another class of monomers include an alkylene or a polyalkylene glycoldiacrylate prepared from an alkylene glycol of 2 to 15 carbons or apolyalkylene ether glycol of 1 to 10 ether linkages, and those disclosedin U.S. Pat. No. 2,927,024, e.g., those having a plurality of additionpolymerizable ethylenic linkages particularly when present as terminallinkages. This class of monomers wherein at least one and preferablymost of such linkages are conjugated with a double bonded carbon,including carbon double bonded to carbon and to such heteroatoms asnitrogen, oxygen and sulfur, are suitable in some embodiments. Alsosuitable are such monomeric materials wherein the ethylenicallyunsaturated groups, especially the vinylidene groups, are conjugatedwith ester or amide structures.

Further examples of monomers can be found in Chen U.S. Pat. No.4,323,636; Fryd et al., U.S. Pat. No. 4,753,865; Fryd et al., U.S. Pat.No. 4,726,877 and Feinberg et al., U.S. Pat. No. 4,894,315.

The photoinitiator can be any single compound or combination ofcompounds that is sensitive to actinic radiation to form a species thatwill initiate either free radical crosslinking and/or polymerizationreactions. The photoinitiator for the main or overall exposure (as wellas an optional post-exposure and backflash) is sensitive to visible orultraviolet radiation, between 310 to 400 nm, and preferably 345 to 365nm. A photoinitiator or photoinitiator system that generates freeradicals based on photoinduced hydrogen abstraction mechanism isparticularly suitable for use in the present invention. Thesephotoinitiators are particularly effective at capturing the actinicradiation impinging the photosensitive layer and initiatingpolymerization in the exposed portions of the layer to allow forcomplete or substantially complete photochemical reinforcement to occur.Photoinitiators based on hydrogen abstraction may also be referred to aNorrish type II photoinitiators. Electrically excited carbonyl compoundsare hydrogen abstractors that are considered Norrish type IIphotoinitiators. In one embodiment of the hydrogen abstractionmechanism, the excited state of the carbonyl compound abstracts ahydrogen from an appropriate substrate or hydrogen donor compound, knownas a coinitiator, to form a ketyl radical (derived from the carbonylcompound) and a radical derived from the coinitiator. The coinitiatorradical can either initiate polymerization or abstract a hydrogen atomfrom another substrate to produce a secondary radical that initiatespolymerization. Carbonyl compounds suitable as the Norrish type IIphotoinitiator include aromatic ketones and quinones, such as, forexample, benzophenones, ketosulphones, thioxanthones, 1,2-diketones,anthraquinone, fluorenones, xanthones, acetophenone derivatives, benzoinethers, benzl ketals, phenylglyoxylates, mono- and bis-acylphosphine.Suitable coinitiators include thiols, aldehydes, secondary alcohols,primary amines, secondary amines, and tertiary amines. One embodiment ofa suitable coinitiator is an aromatic-amine disclosed in formula I byRomagnano et al. in WO 2007/017298, which is hereby incorporated byreference. Examples of suitable Norrish type II photoinitiator systemscomposed of the Norrish type II photoinitiator and coinitiator include,but are not limited to, anthraquinone and a hydrogen donor; benzophenoneand tertiary amines; Michler's ketone alone and with benzophenone;thioxanthones and a coinitiator; and 3-ketocourmarins and a coinitiator.One embodiment of a suitable photoinduced hydrogen abstractionphotoinitiator and coinitiator system is disclosed by Romagnano et al.in WO 2007/017298, which is hereby incorporated in its entirety byreference. The Norrish type II photoinitiator is present in thephotosensitive composition in an amount of 0.1 to 15% by weight, basedon the weight of the photosensitive composition. In one embodiment, thecoinitiator may be present in the photosensitive composition in anamount of 0.1 to 15% by weight, based on the weight of thephotosensitive composition. In another embodiment, the coinitiator neednot be present. In an embodiment in which the binder includes sufficientpendant alkyl groups of 1 to 6 carbon atoms, such as for example,methyl, ethyl, and tert-butyl, the binder can assist with thecoinitiator or act as the coinitiator in donating hydrogen upon reactionwith the Norrish type II photoinitiator.

While the Norrish type II photoinitiators will drive the bulk of thephotopolymerization reaction for the photosensitive element, thefollowing optional photoinitiators may also aid in photohardening theelement. Optionally, the photosensitive composition may include a secondphotoinitiator or photoinitiator system that generates free radicalsbased on photofragmentation mechanism, which may be referred to asNorrish type I photoinitiators. Suitable initiators forphotofragmentation mechanism include, but are not limited to, peroxides,azo compounds, benzoin derivatives, derivatives of acetophenone,ketoxime esters of benzoin, triazines, and biimidazoles. As is known tothose skilled in the art, acetophenone derivatives can be at the core ofNorrish type I and Norrish type II photoinitiator reactions.

Optionally, a second photoinitiator sensitive to radiation between 220to 300 nm, preferably 245 to 265 nm, may optionally be present in thephotosensitive composition. After treating or engraving, a plate can befinished with radiation between 220 to 300 nm to detackify the reliefsurfaces. The second photoinitiator decreases the finishing exposuretime necessary to detackify the plate. The second photoinitiator can beeffective in photosensitive elements having transmission to actinicradiation of less than 20% since the detackifying reaction occurs at anexterior surface (i.e., contact surface) of the printing form. Theseoptional photoinitiators may be independently present in amounts from0.001% to 10.0% based on the weight of the photopolymerizablecomposition.

The photosensitive composition includes reinforcing particles thatenhance the mechanical strength of the printing element. The mechanicalstrength of the element can be enhanced for the purpose of laserengraving and/or for end-use purposes to withstand the rigors of certainprinting applications associated with the use of materials such as inksor substrates, or processes for printing. The presence of thereinforcing particles in the photosensitive layer can also enhance thesolvent resistance of the printing element. Reinforcing particles ofgraphene and/or carbon nanotubes in the photosensitive layersurprisingly provide a barrier to solvent action, and significantlyimprove resistance to swelling by solvents and even potent solvents inthe photosensitive element and in the resulting printing form.Reinforcing particles may also be referred to herein as reinforcingparticulate and reinforcing agents.

The photosensitive composition includes reinforcing particles that areselected from the group consisting of graphene, carbon nanotubes, andcombinations thereof. Graphene is a single planar sheet of sp²-bonedcarbon atoms that is densely packed into a benzene-ring structure andconsidered an aromatic. Graphenes are two dimensional counterparts ofthree dimensional graphite. Carbon nanotubes are considered graphenesheets rolled up into nanometer-sized cylinders. The reinforcingparticles of graphene and/or carbon nanotubes should be suitablydispersed within the layer formed from the photosensitive composition,which can be accomplished with the aid of a dispersing agent. Thegraphenes and/or carbon nanotubes have a particle size which can rangefrom about 100 nm to about 10 micron. In other embodiments, the particlesize can range from about 100 nm to about 5000 nm. In some embodiments,the particle size of graphenes can range from about 200 nm to about 7 to10 micron in length. In some embodiments, the particle size of carbonnanotubes can range from about 300 to 800 nm in length.

Optionally, the graphene and/or carbon nanotubes can be incorporatedwith a component that allows for dispersing of the particles in thecomposition and uniform distribution of the particles in thephotosensitive layer of the element. The layer containing the grapheneand/or carbon nanotubes can be prepared by conventional methods bycombining the particles with the one or more components. Generally it isnot suitable to combine the graphene and/or carbon nanotubes that is inparticulate form in a liquid-like component due to the difficulty indispersing the graphene and/or carbon nanotubes in solution. Since thegraphene and/or carbon nanotubes are in particulate form, a dispersantcan be added in order to disperse the graphene and/or carbon nanotubesparticles and avoid flocculation and agglomeration. Dispersants suitablefor use are not limited, provided that the dispersant can uniformlydistribute the reinforcing particles in the layer, and is sufficientlycompatible with the binder and other components in the photosensitivelayer to the extent that a clear, non-cloudy photosensitive layer isproduced. A wide range of dispersants are commercially available. Oneembodiment of suitable dispersants are the A-B dispersants generallydescribed in “Use of A-B Block Polymers as Dispersants For Non-aqueousCoating Systems” by H. K. Jakubauskas, Journal of Coating Technology,Vol. 58; Number 736; pages 71-82. Useful A-B dispersants are disclosedin U.S. Pat. Nos. 3,684,771; 3,788,996; 4,070,388; and 4,032,698. Thedispersant is generally present in an amount of about 0.1 to 10% byweight, based on the total weight of the photosensitive composition. Inanother embodiment, the graphene and/or carbon nanotubes is incorporatedwith a polymeric binder. The graphene and/or carbon nanotubes can becombined with a polymeric binder which can be selected from the bindersdescribed as suitable for use in the photosensitive layer and theadditional layers, and which can be the same or different from thebinder used in the photosensitive layer and/or the additional layers. Apreferred method for preparing the composition containing the grapheneand/or carbon nanotubes is to precompound the graphene and/or carbonnanotubes with a portion of the total amount of binder, and then add theremaining portion of the binder to the precompounded mixture. Adding ofthe precompounding mixture to the remaining portion of the binderencompasses diluting, mixing, and/or blending. At any point in theprecompounding, a solvent can be used for dispersing the materials usedin the diluting, mixing, and/or blending steps. The weight ratio of theprecompounded mixture to the remaining binder portion in someembodiments can be 1:10000 to 1:100, and in other embodiments can be1:1000 to 1:10. This is done to ensure that the graphene and/or carbonnanotubes is well dispersed in the binder and uniformly distributed inthe layer.

The reinforcing particles of graphene and carbon nanotubes are dark,that is, the particles by themselves or in combination with thephotosensitive composition are devoid or partially devoid of light anddo not or only partially receive, reflect, or transmit light. Thepresence of the reinforcing particles of graphene and/or carbonnanotubes provide the photosensitive layer with a transmittance toactinic radiation of less than 20%, and in some embodiments of less than10%. The presence of the graphene and/or carbon nanotubes as reinforcingparticles provide the photosensitive layer with a transmittance toactinic radiation of less than 10% in some embodiments, less than 5% inother embodiments, and less than 1% in yet other embodiments. Thereinforcing particles of graphene and/or carbon nanotubes are present inthe photosensitive layer in an amount greater than 0.4% by weight insome embodiments, greater than 0.8% by weight in other embodiments, andgreater than 1% by weight in yet other embodiments, based on the weightof the photosensitive composition. In some embodiments, the reinforcingparticles of graphene and/or carbon nanotubes are present in amountsfrom 1 to 5% by weight of the composition.

Optionally, the photosensitive composition may also include, with thegraphene and/or carbon nanotubes, additional reinforcing particles. Anytype of additional reinforcing particles can be employed in the presentinvention, provided that the particles can be suitably dispersed withinthe layer formed from the photosensitive composition. Optionally, theadditional reinforcing particles can also be incorporated into thephotosensitive composition with the aid of a dispersing agent asdescribed above. The reinforcing particulate can be organic, inorganic,mixtures of organic and inorganic compounds, or multicomponent. Thereinforcing particulate can include additives. The reinforcing particlesare preferably a fine powder having a wide range of particle sizes.

For additional reinforcing particles that are dark, the presence of thereinforcing particles in the photosensitive layer at as little as 0.4%by weight can render the photosensitive layer opaque enough to preventor significantly reduce (to less than 10%, and in some embodiments toless than 20%) the penetration of actinic radiation, i.e., ultravioletradiation, necessary for photochemical reinforcement of the layer. Thephotosensitive layer may have less than 10% transmittance at higherloading (e.g., greater than 0.4% by weight in some embodiments, greaterthan 1% by weight in other embodiments, and greater than 10% by weightfor yet other embodiments) of the additional reinforcing particles thatare not dark in the photosensitive layer. The additional reinforcingparticles are present in the photosensitive layer in an amount greaterthan 0.4% by weight in some embodiments, greater than 0.8% by weight inother embodiments, and greater than 1% by weight in yet otherembodiments, based on the weight of the photosensitive composition. Insome embodiments, the additional reinforcing particles are present inamounts from 1 to 5% by weight, 5 to 20% by weight, and from 1 to about25% by weight of the composition.

The engraving sensitivity of the photosensitive layer is increased bythe presence of the reinforcing particles which is sensitive to thewavelength of laser radiation used for engraving, which in someembodiments is 9 and 12 micrometers. In one embodiment, the sensitivityof the reinforcing particles is matched to the wavelength of laserradiation used for engraving which is typically a CO₂ laser. In someembodiments, the reinforcing particles absorb infrared radiation at 9 to12 micrometers. Increased engraving sensitivity of the elastomeric layerresults in increasing the speed of engraving (compared to elastomericmaterials without reinforcing particles) and reducing the tackiness ofthe debris generated by engraving. The additive can also enhance themechanical properties of the elastomeric layer, such as tensilestrength, stiffness, tear resistance, and abrasion resistance.

Increasing the amount of reinforcing particles causes a concomitantimprovement in the laser engravability and the mechanical properties ofthe photosensitive element (and printing form) until a maximum isreached which represents the optimum loading for a particularcomposition. Beyond this point, the properties of the photosensitivelayer will deteriorate. The effectiveness of the reinforcing particlesalso depends on the particle size and the tendency of the material toagglomerate or form chains. In general, tensile strength, abrasion andtear resistance, hardness, and toughness increase with decreasingparticle size.

The effectiveness of the reinforcing particles also depends on theparticle size and the tendency of the material to agglomerate or formchains. In general, tensile strength, abrasion and tear resistance,hardness, and toughness increase with decreasing particle size. Carbonblack having particle size of 200 to 500 angstroms in diameter issuitable. For other additional reinforcing particles, particle sizes upto a few micrometers in diameter can be used. In some embodiments, theparticles are less than 1 micrometer. Reinforcing particles that arenanometer (10⁻⁹) sized particles offer a particular advantage overconventional micron sized reinforcing particles in that the nanometerparticles are so small that they do not reduce the penetration orscatter the actinic radiation that exposes the element, and thereby mayprovide the capability for incorporating higher loading levels of thereinforcing particles into the photosensitive layer.

Examples of materials suitable for the additional reinforcing particlesinclude, but are not limited to, carbon black, graphite, and furnaceblack. Other examples of additional reinforcing particles include, butare not limited to, TiO₂, calcium carbonate, calcium silicate, bariumsulfate, mica, zinc oxide, magnesium oxide, titanium oxide, aluminum,and alumina. Additional reinforcing particles also encompass materialshaving silica-oxygen (Si—O bond) functionality or phosphorus-oxygen (P—Obond) functionality. Suitable materials with silica-oxygen single-bondfunctionality include particulate inorganic filler materials such assilicas, clays, talcs, mica, and silicates, for example, calciumsilicate and zirconium silicate. Suitable materials containingphosphorous-oxygen single bond functionality include particulates suchas aromatic phosphates, aromatic phosphites and aromatic phosphonates.

Other examples of additional reinforcing particles includes particleshaving color such as pigment particles, toner particles, mixtures ofpigment particles, mixtures of toner particles and mixtures of pigmentand toner particles. Pigment particles as reinforcing particles can bein crystallite form or may be aggregates. Pigment particles suitable foruse can be any provided the pigment provides the desired characteristicsto the photosensitive element such as an increase in mechanicalreinforcement. Some suitable additional reinforcing particles includedark blue pigments, and dark brown pigments. Further examples of pigmentparticles include, but are not limited to, Toluidine Red (C.I. PigmentRed 3), Toluidine Yellow (C.I. Pigment Yellow 1), copper phthalocyaninecrystals, quinacridone crystals, Toluidine Red YW, Watchung Red BW (C.I.Pigment Red 48), Toluidine Yellow GW (C.I. Pigment Yellow 1), MonastralBlue BW (C.E. Pigment Blue 15), Monastral Green BW (C.I. Pigment Green7), Pigment Scarlet (C.I. Pigment Red 60), Auric Brown (C.I. PigmentBrown 6), Monastral Green (Pigment Green 7) and Monastral Maroon B andMonastral Orange. Also suitable as pigment particles are copperchromite, chromium oxides and cobalt chrome aluminate; and metals suchas aluminum, copper or zinc, and alloys of bismuth, indium, and copper.

Toner particles are pigmented organic resin particles which comprisepigment particles finely dispersed in a resin matrix which is thenground to the desired particle size. Pigmented organic resin particlesare described in Chu and Manger in U.S. Pat. No. 3,620,726; Vesce inU.S. Pat. No. 2,649,382; and Gray in U.S. Pat. No. 3,909,282 which arehereby incorporated by reference. Pigments suitable for use in the tonerparticles can be any including those mentioned above, provided the tonerprovides the desired opacity to the particles as well as other desiredcharacteristics. Suitable resin matrices include, but not limited to,polyvinyl chloride, cellulose acetate, cellulose acetate butyrate,polystyrene, polymethyl methacrylate. Also useful are water solublepolymer matrices, for example, polyvinyl alcohol, methyl cellulose,carboxymethyl cellulose. The particular matrix being used depending onthe mechanical means of processing the toner down to the desiredeffective particle size, desired opacity, and desired ablationsensitivity. Toners suitable for use as the powder layer are not limitedand can include toners used for pre-press proofing systems as well aselectroconductive toners used in xerographic copy systems. Particularlypreferred toners are black toners sold by DuPont as Cromalin® blacktoners, e.g., Cromalin® type KK6 black toner e.g. a carbon black andcellulose acetate blend.

In one embodiment, the photosensitive composition includes reinforcingparticles selected from the group consisting of graphene, carbonnanotubes, and combinations thereof. In another embodiment, thephotosensitive composition includes reinforcing particles selected fromthe group consisting of graphene, carbon nanotubes, and combinationsthereof; and, one type of additional reinforcing particles. In otherembodiments, the photosensitive composition includes reinforcingparticles selected from the group consisting of graphene, carbonnanotubes, and combinations thereof; and, more than one type ofadditional reinforcing particles. Some embodiments of these reinforcingparticles of graphene and/or carbon nanotubes alone can provide desiredimprovements in mechanical reinforcement and solvent resistance of theelement, and render the layer sufficiently opaque (i.e., the layerhaving a transmittance to actinic radiation less than 10%, and in otherembodiments having a transmittance to actinic radiation less than 20%)that the Norrish type II photoinitiator suitably drives thepolymerization reaction for photochemical reinforcement. In otherembodiments, these reinforcing particles of graphene and/or carbonnanotubes alone may provide desired improvements in mechanicalreinforcement and solvent resistance of the element, but do not renderthe layer sufficiently opaque (i.e., the layer has a transmittance toactinic radiation greater than about 10%, and in some embodimentsgreater than 20%). In this case it may be desirable to include dyes orother materials to render the layer suitably opaque.

Additional additives to the photosensitive layer include plasticizers,colorants, dyes, processing aids, antioxidants, and antiozonants.Plasticizers, which may sometimes also include monomeric functionality,can include acrylated liquid polyisoprenes, acrylated liquidpolybutadienes, liquid polyisoprenes with high vinyl content, and liquidpolybutadienes with high vinyl content, (that is, content of 1-2 vinylgroups is greater than 20% by weight). Processing aids may be suchthings as low molecular weight polymers compatible with the binder.Antiozonants include hydrocarbon waxes, norbornenes, and vegetable oils.Suitable antioxidants include alkylated phenols, alkylated bisphenols,polymerized trimethyldihydroquinone, and dilauryl thiopropinoate.

The thickness of the solid photosensitive layer can vary over a widerange depending upon the type of printing form desired. In oneembodiment, the photosensitive layer can have a thickness from about0.015 inch to about 0.250 inch or greater (about 0.038 to about 0.64 cmor greater), and preferably about 0.020 to 0.155 inch (0.5 mm to 3.9mm). In another embodiment, the photosensitive layer can have athickness from about 0.002 inch to 0.025 inch (0.051 to 0.64 mm), andpreferably 0.005 to 0.020 inch (0.13 to 0.5 mm).

The photosensitive element may optionally include a support adjacent thelayer of the photosensitive composition. The support can be composed ofany material or combination of materials that is conventionally usedwith photosensitive elements used to prepare printing forms. In someembodiments, the support is transparent to actinic radiation toaccommodate “backflash” exposure through the support. Examples ofsuitable support materials include polymeric films such those formed byaddition polymers and linear condensation polymers, transparent foamsand fabrics, such as fiberglass. Under certain end-use conditions,metals such as aluminum, steel, and nickel, may also be used as asupport, even though a metal support is not transparent to radiation. Apreferred support is a polyester film; particularly preferred ispolyethylene terephthalate film. The support may be in sheet form or incylindrical form, such as a sleeve. The sleeve can be formed of anymaterial or combination of materials conventionally used in formingsleeves for printing. The sleeve can have a single layer, multi-layer,composite, or unitary structure. Sleeves made of polymeric films aretypically transparent to actinic radiation and thereby accommodatebackflash exposure for building a floor in the cylindrical printingelement. Multiple layered sleeves may include an adhesive layer or tapebetween the layers of flexible material, such as disclosed in U.S. Pat.No. 5,301,610. The sleeve may also be made of non-transparent, actinicradiation blocking materials, such as nickel or glass epoxy. The sleevemay be composed of one or more layers of a resin composition, which canbe the same or different, and have fillers and/or fibers incorporatedtherein. Materials suitable as the resin composition are not limited,examples of which include, epoxy resins; polystyrene and polyvinylresins, such as polyvinyl chloride and polyvinyl acetate; phenolicresins; and aromatic amine-cured epoxy resins. The fibers used in theresin composition are not limited and can include, for example, glassfibers, aramid fibers, carbon fibers, metal fibers, and ceramic fibers.Fibers incorporated with the sleeve can include continuous, woven,and/or wound materials. The support formed of a resin compositionreinforced with fiber is an example of a composite sleeve. The supporttypically has a thickness from 0.002 to 0.050 inch (0.0051 to 0.127 cm).A preferred thickness for the sheet form support is 0.003 to 0.016 inch(0.0076 to 0.040 cm). The sleeve can have a wall thickness from about0.01 and about 6.35 mm or more. In some embodiments, the sleeve has awall thickness between about 0.25 and 0.80 mm. In some embodiments, thesleeve has a wall thickness between about 10 to 80 mils (0.25 to 2.0mm), and in other embodiments 10 to 40 mils (0.25 to 1.0 mm). In yetother embodiments, the sleeve has a thickness between about 1 and 3 mm.

Optionally, the element can include an adhesive layer between thesupport and the photopolymerizable layer, or a surface of the supportthat is adjacent the photopolymerizable layer has an adhesion-promotingsurface. The adhesive layer on the surface of the support can be asubbing layer of an adhesive material or primer or an anchor layer asdisclosed in U.S. Pat. No. 2,760,863 to give strong adherence betweenthe support and the photopolymerizable layer. The adhesive compositionsdisclosed in Burg, U.S. Pat. No. 3,036,913 are also effective.Alternatively, the surface of the support on which thephotopolymerizable layer resides can be treated to promote adhesionbetween the support and the photopolymerizable layer, withflame-treatment or electron-treatment, e.g., corona-treated. Further,the adhesion of the photopolymerizable layer to the support can beadjusted by exposing the element to actinic radiation through thesupport as disclosed by Feinberg et al. in U.S. Pat. No. 5,292,617.

In one embodiment, the photosensitive element includes the support and alayer of the photosensitive composition on the support. The layer of thephotosensitive composition is primarily reinforced by the inclusion ofthe reinforcing particles to enhance mechanical properties of theelement and resultant printing form. The presence of the reinforcingparticles can also enhance engraving efficiency of the element by laserradiation. The photosensitive layer is also reinforced photochemicallyby exposure to actinic radiation to affect photohardening in depth. Thephotosensitive layer includes a Norrish type II photoinitiator that iseffective at capturing actinic radiation and initiating polymerizationthrough the exposed portions of the particulate filled layer to allowfor complete or substantially complete photochemical reinforcement. Thecomposition layer is at least one layer on the substrate capable ofbeing treated to form a relief suitable for printing. In the presentinvention, treating encompasses various methods of forming the reliefprinting surface from the photosensitive element including, applicationof one or more solutions and/or heat to an imagewise exposedphotosensitive element to remove uncured (i.e., unpolymerized material)from the layer to form the relief printing surface; and, exposing thephotosensitive element to actinic radiation to photochemically reinforcethe element and exposing with laser radiation to selectively remove thereinforced material to form the relief printing surface. In oneembodiment, the photosensitive element is an elastomeric printingelement suitable for use as a flexographic printing form. In anotherembodiment, the photosensitive element can be transformed into aprinting form for gravure-like printing.

The photosensitive element includes at least one photosensitive layerthat contains the reinforcing particles that provide the layer with atransmittance to actinic radiation of less than 20% and are selectedfrom the group consisting of graphene, carbon nanotubes, andcombinations thereof. The photosensitive element can be a bi- ormulti-layer construction, wherein the additional layer/s can bephotosensitive (or can become photosensitive), or non-photosensitive.The additional layer/s can have the same, or substantially the same, ordifferent composition as the photosensitive composition that containsthe reinforcing particles. In some embodiments, the photosensitiveelement may include an intermediate layer between a support and a toplayer of the photosensitive composition containing the reinforcingparticles. In some embodiments, the intermediate layer may provide thephotosensitive element with desired bulk properties for end-use as theprinting form. For example in one embodiment of photosensitive elementsfor use as flexographic printing forms, the intermediate layer may be anelastomeric non-photosensitive layer that provides the printing formwith desired Shore A hardness, resilience, and/or compressibility, andthe top layer is the photosensitive layer containing the reinforcingparticles. The one or more additional layers can undergo the sametreatment steps, or can remain unaffected by the treatment steps, thatthe photosensitive layer containing the reinforcing particles undergoes.

The photosensitive element may include one or more additional layers onor adjacent the photosensitive layer. In most embodiments the one ormore additional layers are on a side of the photosensitive layeropposite the support. Examples of additional layers include, but are notlimited to, a release layer, a capping layer, an elastomeric layer, abarrier layer, and combinations thereof. The one or more additionallayers can be removable, in whole or in part, during treatment. One ormore of the additional layers may cover or only partially cover thephotosensitive composition layer. An example of an additional layerwhich only partially covers the photosensitive composition layer is amasking layer that is formed by imagewise application, e.g., ink jetapplication, of an actinic radiation blocking material or ink.

The release layer protects the surface of the composition layer andenables the easy removal of a mask used for the imagewise exposure ofthe photosensitive element. Materials suitable as the release layer arewell known in the art. Suitable compositions for the capping layer andmethods for forming the layer on the element are disclosed aselastomeric compositions in a multilayer cover element described inGruetzmacher et al., U.S. Pat. Nos. 4,427,759 and 4,460,675. Theelastomeric capping layer is similar to the photosensitive layer in thatafter imagewise exposure the elastomeric capping layer is at leastpartially removable by treating. The elastomeric capping layer includesan elastomeric binder, which can be the same or different from theelastomeric binder described above, and optionally, one or moremonomers, photoinitiator or photoinitiator system, and other additivesas described for the photosensitive layer. The elastomeric layer orcapping layer can be photosensitive itself, that is, contain monomer andinitiator, or it can become photosensitive when in contact with thephotopolymerizable layer. The composition of the elastomeric cappinglayer can be the same as, or substantially the same as, or differentfrom the composition of the adjacent photopolymerizable layer. In someembodiments, the elastomeric layer includes with an elastomeric binder,a Norrish type II photoinitiator and reinforcing agents selected fromthe group consisting of graphene, carbon nanotubes, and combinationsthereof. The elastomeric capping layer is solid that generally forms amonolithic structure with the adjacent photopolymerizable layer. Thethickness of the elastomeric capping layer is typically between about0.001 inch to about 0.010 inch (0.025 to 0.25 mm).

The photosensitive element of the present invention may further includea temporary coversheet on top of the uppermost layer of thephotosensitive element. One purpose of the coversheet is to protect theuppermost layer of the photosensitive element during storage andhandling. Depending upon end use, the coversheet may or may not beremoved prior to imaging, but is removed prior to development. Suitablematerials for the coversheet are well known in the art.

The photosensitive composition can be prepared by employing a variety oftechniques that are well known in the art. One method which can be usedis to mix the components (that is, binder, initiator, monomer withsilica particles, and other ingredients) in an extruder and then extrudethe mixture as a hot melt onto a support. It is preferred that theextruder be used to perform the functions of melting, mixing,deaerating, and filtering the composition. To achieve uniform thickness,the extrusion step can be advantageously coupled with a calendering stepin which the hot mixture is calendered between two sheets or between oneflat sheet and a release roll. Alternately, the material can beextruded/calendered onto a temporary support and later laminated to thedesired final support. The elements can also be prepared by compoundingthe components in a suitable mixing device and then pressing thematerial into the desired shape in a suitable mold. The material isgenerally pressed between the support and the coversheet. The moldingstep can involve pressure and/or heat. The coversheet may include one ormore of the additional layers which transfer to the photopolymerizablelayer when the photosensitive element is formed.

In one embodiment, the photosensitive element is prepared for treatmentby imagewise exposing the element to actinic radiation. After imagewiseexposure, the photosensitive element contains cured portions in theexposed areas of the radiation curable composition layer and uncuredportions in the unexposed areas of the radiation curable compositionlayer. Imagewise exposure is carried out by exposing the photosensitiveelement through an image-bearing mask. The image-bearing mask may be ablack and white transparency or negative containing the subject matterto be printed, or an in-situ mask formed on the composition layer bymeans known in the art. Imagewise exposure can be carried out in avacuum (frame) or may be conducted in the presence of atmosphericoxygen. On exposure, the transparent areas of the mask allow additionpolymerization or crosslinking to take place, while the actinicradiation opaque areas remain uncrosslinked. Exposure is of sufficientduration to crosslink the exposed areas down to the support or to a backexposed layer (floor). Imagewise exposure time is typically much longerthan backflash time, and ranges from a few to tens of minutes.

For direct-to-plate image formation as disclosed in U.S. Pat. No.5,262,275; U.S. Pat. No. 5,719,009; U.S. Pat. No. 5,607,814; U.S. Pat.No. 5,506,086; U.S. Pat. No. 5,766,819; U.S. Pat. No. 5,840,463; U.S.Pat. No. 6,238,837; U.S. Pat. No. 6,558,876; and U.S. Pat. No. 6,773,859the image-bearing mask is formed in-situ with the laser radiationsensitive layer using an infrared laser exposure engine. The imagewiselaser exposure can be carried out using various types of infraredlasers, which emit in the range 750 to 20,000 nm, preferably in therange 780 to 2,000 nm. Diode lasers may be used, but Nd:YAG lasersemitting at 1060 nm are preferred. To the extent that the infrared laserexposure engine can be controlled or tuned such that the infraredsensitive layer can be removed from or transferred to the photosensitivelayer without detrimental impact to the underlying photopolymerizablelayer which is also dark, these systems are suitable.

It is also contemplated that in-situ digital mask formation can beaccomplished by imagewise application of the radiation opaque materialin the form of inkjet inks. Imagewise application of an ink-jet ink canbe directly on the photopolymerizable layer or disposed above thephotopolymerizable layer on another layer of the photosensitive element.Another contemplated method that digital mask formation can beaccomplished is by creating the mask image of the radiation opaque layeron a separate carrier and then transferring with application of heatand/or pressure to the surface of the photopolymerizable layer oppositethe support. The photopolymerizable layer is typically tacky and willretain the transferred image. The separate carrier can then be removedfrom the element prior to imagewise exposure. The separate carrier mayhave a radiation opaque layer that is imagewise exposed to laserradiation to selectively remove the radiation opaque material and formthe image.

Actinic radiation sources encompass the ultraviolet, visible andinfrared wavelength regions. The suitability of a particular actinicradiation source is governed by the photosensitivity of the initiatorand the at least one monomer used in preparing the flexographic printingplates from the photosensitive element. The preferred photosensitivityof most common flexographic printing plates is in the UV and deepvisible area of the spectrum, as they afford better room-lightstability. The portions of the composition layer that are exposed toradiation chemically cross-link and cure. The portions of thecomposition layer that are unirradiated (unexposed) are not cured andhave a lower melting or liquefying temperature than the cured irradiatedportions. The imagewise exposed photosensitive element is then ready fortreatment to remove unpolymerized areas in the photopolymerizable layerand thereby form a relief image areas of the image.

An overall back exposure through the support side, a so-called backflashexposure, may be conducted to polymerize a predetermined thickness ofthe photopolymer layer adjacent the support. The backflash exposure maybe conducted before, after, or even during other imaging steps, theimagewise exposure. This polymerized portion of the photopolymer layeris designated a floor. The floor provides improved adhesion between thephotopolymerizable layer and the support, helps highlight dot resolutionand also establishes the depth of the plate relief. The floor thicknessvaries with the time of exposure, exposure source, etc. This exposuremay be done diffuse or directed. All radiation sources suitable forimagewise exposure may be used. The exposure is generally for 10 secondsto 30 minutes.

Following overall exposure to UV radiation through the mask, thephotosensitive printing element is treated to remove unpolymerized areasin the photopolymerizable layer and thereby form a relief image. Thetreating step removes at least the photopolymerizable layer in the areaswhich were not exposed to actinic radiation, i.e., the unexposed areasor uncured areas, of the photopolymerizable layer. Except for theelastomeric capping layer, typically the additional layers that may bepresent on the photopolymerizable layer are removed or substantiallyremoved from the polymerized areas of the photopolymerizable layer. Forphotosensitive elements having an in-situ mask, the treating step alsoremoves the mask image (which had been exposed to actinic radiation) andthe underlying unexposed areas of the photopolymerizable layer.

In one embodiment, treatment of the photosensitive element includes (1)“wet” development wherein the photopolymerizable layer is contacted witha suitable developer solution to washout unpolymerized areas and (2)“dry” development wherein the photosensitive element is heated to adevelopment temperature which causes the unpolymerized areas of thephotopolymerizable layer to melt or soften or flow and then are removed.Dry development may also be called thermal development. It is alsocontemplated that combinations of wet and dry treatment can be used toform the relief.

Wet development is usually carried out at about room temperature. Thedevelopers can be organic solvents, aqueous or semi-aqueous solutions,and water. The choice of the developer will depend primarily on thechemical nature of the photopolymerizable material to be removed.Suitable organic solvent developers include aromatic or aliphatichydrocarbon and aliphatic or aromatic halohydrocarbon solvents, ormixtures of such solvents with suitable alcohols. Other organic solventdevelopers have been disclosed in published German Application 38 28551. Suitable semi-aqueous developers usually contain water and a watermiscible organic solvent and an alkaline material. Suitable aqueousdevelopers usually contain water and an alkaline material. Othersuitable aqueous developer combinations are described in U.S. Pat. No.3,796,602.

Development time can vary, but it is preferably in the range of about 2to about 25 minutes. Developer can be applied in any convenient manner,including immersion, spraying and brush or roller application. Brushingaids can be used to remove the unpolymerized portions of the element.Washout can be carried out in an automatic processing unit which usesdeveloper and mechanical brushing action to remove the uncured portionsof the plate, leaving a relief constituting the exposed image and thefloor.

Following treatment by developing in solution, the relief printingplates are generally blotted or wiped dry, and then more fully dried ina forced air or infrared oven. Drying times and temperatures may vary,however, typically the plate is dried for 60 to 120 minutes at 60° C.High temperatures are not recommended because the support can shrink andthis can cause registration problems.

Treating the element thermally includes heating the photosensitiveelement having at least one photopolymerizable layer (and the additionallayer/s) to a temperature sufficient to cause the uncured portions ofthe photopolymerizable layer to liquefy, i.e., soften or melt or flow,and removing the uncured portions. The layer of the photosensitivecomposition is capable of partially liquefying upon thermal development.That is, during thermal development the uncured composition must softenor melt at a reasonable processing or developing temperature. If thephotosensitive element includes one or more additional layers on thephotopolymerizable layer, it is preferred that the one or moreadditional layers are also removable in the range of acceptabledeveloping temperatures for the photopolymerizable layer. Thepolymerized areas (cured portions) of the photopolymerizable layer havea higher melting temperature than the unpolymerized areas (uncuredportions) and therefore do not melt, soften, or flow at the thermaldevelopment temperatures. The uncured portions can be removed from thecured portions of the composition layer by any means including air orliquid stream under pressure as described in U.S. publication2004/0048199 A1, vacuum as described in Japanese publication 53-008655,and contacting with an absorbent material as described in U.S. Pat. No.3,060,023; U.S. Pat. No. 3,264,103; U.S. Pat. No. 5,015,556; U.S. Pat.No. 5,175,072; U.S. Pat. No. 5,215,859; U.S. Pat. No. 5,279,697; andU.S. Pat. No. 6,797,454. A preferred method for removing the uncuredportions is by contacting an outermost surface of the element to anabsorbent surface, such as a development medium, to absorb or wick awayor blot the melt portions.

The term “melt” is used to describe the behavior of the unirradiated(uncured) portions of the composition layer subjected to an elevatedtemperature that softens and reduces the viscosity to permit absorptionby the absorbent material. The material of the meltable portion of thecomposition layer is usually a viscoelastic material which does not havea sharp transition between a solid and a liquid, so the processfunctions to absorb the heated composition layer at any temperatureabove some threshold for absorption in the development medium. Thus, theunirradiated portions of the composition layer soften or liquefy whensubjected to an elevated temperature. However throughout thisspecification the terms “melting”, “softening”, and “liquefying” may beused to describe the behavior of the heated unirradiated portions of thecomposition layer, regardless of whether the composition may or may nothave a sharp transition temperature between a solid and a liquid state.A wide temperature range may be utilized to “melt” the composition layerfor the purposes of this invention. Absorption may be slower at lowertemperatures and faster at higher temperatures during successfuloperation of the process.

The thermal treating steps of heating the photosensitive element andcontacting an outermost surface of the element with development mediumcan be done at the same time, or in sequence provided that the uncuredportions of the photopolymerizable layer are still soft or in a meltstate when contacted with the development medium. The at least onephotopolymerizable layer (and the additional layer/s) are heated byconduction, convection, radiation, or other heating methods to atemperature sufficient to effect melting of the uncured portions but notso high as to effect distortion of the cured portions of the layer. Theone or more additional layers disposed above the photopolymerizablelayer may soften or melt or flow and be absorbed as well by thedevelopment medium. The photosensitive element is heated to a surfacetemperature above about 40° C., preferably from about 40° C. to about230° C. (104-446° F.) in order to effect melting or flowing of theuncured portions of the photopolymerizable layer. By maintaining more orless intimate contact of the development medium with thephotopolymerizable layer that is molten in the uncured regions, atransfer of the uncured photosensitive material from thephotopolymerizable layer to the development medium takes place. Whilestill in the heated condition, the development medium is separated fromthe cured photopolymerizable layer in contact with the support layer toreveal the relief structure. A cycle of the steps of heating thephotopolymerizable layer and contacting the molten (portions) layer withthe development medium can be repeated as many times as necessary toadequately remove the uncured material and create sufficient reliefdepth. However, it is desirable to minimize the number of cycles forsuitable system performance, and typically the photopolymerizableelement is thermally treated for 5 to 15 cycles. Intimate contact of thedevelopment medium to the photopolymerizable layer (while in the uncuredportions are melt) maybe maintained by the pressing the layer and thedevelopment medium together.

Apparatuses suitable for thermally developing the photosensitive elementare disclosed by Peterson et al. in U.S. Pat. No. 5,279,697, and also byJohnson et al. in U.S. Pat. No. 6,797,454. The photosensitive element inall embodiments is in the form of a plate. However, it should beunderstood that one of ordinary skill in the art could modify each ofthe disclosed apparatuses to accommodate the mounting of thephotosensitive element in the form of a cylinder or a sleeve.

The development medium is selected to have a melt temperature exceedingthe melt or softening or liquefying temperature of the unirradiated oruncured portions of the radiation curable composition and having goodtear resistance at the same operating temperatures. Preferably, theselected material withstands temperatures required to process thephotosensitive element during heating. The development medium may alsobe referred to herein as development material, absorbent material,absorbent web, and web. The development medium is selected fromnon-woven materials, paper stocks, fibrous woven material, open-celledfoam materials, porous materials that contain more or less a substantialfraction of their included volume as void volume. The development mediumcan be in web or sheet form. The development medium should also possessa high absorbency for the molten elastomeric composition as measured bymilligrams of elastomeric composition that can be absorbed per squarecentimeter of the development medium. It is also desirable that fibersare bonded in development mediums containing fibers so that the fibersare not deposited into the form during development. Non-woven nylon andpolyester webs are preferred.

After the treatment step, the photosensitive element can be uniformlypost-exposed to ensure that the photopolymerization process is completeand that the so formed flexographic printing plate will remain stableduring printing and storage. This post-exposure step can utilize thesame radiation source as the imagewise main exposure. Furthermore, ifthe surface of the flexographic printing plate is still tacky,detackification treatments may be applied. Such methods, which are alsocalled “finishing”, are well known in the art. For example, tackinesscan be eliminated by a treatment of the flexographic printing plate withbromine or chlorine solutions. Preferably, detackification isaccomplished by exposure to UV radiation sources having a wavelength notlonger than 300 nm. This so-called “light-finishing” is disclosed inEuropean Published Patent Application 0 017927 and U.S. Pat. No.4,806,506. Various finishing methods may also be combined. Typically,the post-exposure and the finishing exposure are done at the same timeon the photosensitive element using an exposure device that has bothsources of radiation.

In another embodiment, the photosensitive layer is reinforcedphotochemically by overall exposure to actinic radiation to effectphotohardening in depth prior to laser engraving. The layer ofelastomeric composition can be photochemically reinforced during themanufacture of the element or during end-use as part of the formation ofthe element into a printing plate. The radiation source to effectphotohardening of the elastomeric layer should be chosen so that thewavelength emitted matches the sensitive range for the Norrish type IIphotoinitator. This ultraviolet radiation source should furnish aneffective amount of this radiation. In addition to sunlight, suitablehigh energy radiation sources include carbon arcs, mercury-vapor arcs,fluorescent tubes, and sub lamps are suitable. Lasers can be used if theintensity is sufficient only to initiate photohardening, and not toablate material. The exposure time will vary depending upon theintensity and the spectral energy distribution of the radiation, itsdistance from the elastomeric composition, and the nature and amount ofthe elastomeric composition. A removable coversheet can be presentduring the exposure step provided that it is removed after exposure andprior to laser engraving.

After photohardening in the process of making a printing form from thephotosensitive element, the photosensitive element is engraved withlaser radiation. Laser engraving involves the absorption of laserradiation, localized heating and removal of material in threedimensions. The laser engraving process of the invention does notinvolve the use of a mask or stencil. This is because the laser impingesthe reinforced layer to be engraved at or near its focus spot. Thus thesmallest feature that can be engraved is dictated by the laser beamitself. The laser beam and the material to be engraved are in constantmotion with respect to each other, so that each minute area of the plate(pixel) is individually addressed by the laser. The image information isfed into this type of system directly from the computer as digital data,rather than via a stencil. Any pattern of a single image or multipleimages of the same or different images may be engraved.

Factors to be considered when laser engraving include, but are notlimited to, deposition of energy into the depth of the element, thermaldissipation, melting, vaporization, thermally-induced chemical reactionssuch as oxidation, presence of air-borne material over the surface ofthe element being engraved, and mechanical ejection of material from theelement being engraved. Investigative efforts with respect to engravingof metals and ceramic materials with a focused laser beam havedemonstrated that engraving efficiency (the volume of material removedper unit of laser energy) and precision are strongly intertwined withthe characteristics of the material to be engraved and the conditionsunder which laser engraving will occur. Similar complexities come intoplay when engraving elastomeric materials even though such materials arequite different from metals and ceramic materials.

Laser engravable materials usually exhibit some sort of intensitythreshold, below which no material will be removed. Below the thresholdit appears that laser energy deposited into the material is dissipatedbefore the vaporization temperature of the material is reached. Thisthreshold can be quite high for metals and ceramic materials. However,with respect to elastomeric materials it can be quite low. Above thisthreshold, the rate of energy input competes quite well with opposingenergy loss mechanisms such as thermal dissipation. The dissipatedenergy near, though not in, the illuminated area may be sufficient tovaporize to material and, thus, the engraved features become wider anddeeper. This effect is more pronounced with materials having low meltingtemperatures.

Laser engraving can be accomplished by any of various types of infraredlasers emitting infrared radiation in 9 to 12 micrometer wavelength,particularly 10.6 micrometers. The removal of material by the laser isaided by the presence of the additive sensitive to infrared radiation inthe reinforced elastomeric layer which absorbs the radiation energygenerated by the laser. A laser which is particularly suitable forengraving flexographic printing elements is a carbon dioxide laser whichemits at 10.6 micrometer wavelength. Carbon dioxide lasers arecommercially available at a reasonable cost. The carbon dioxide lasercan operate in continuous-wave and/or pulse mode. Capability ofoperating the laser in both modes is desirable since at low or moderateradiation intensities, pulse engraving may be less efficient. Energywhich might heat, even melt the material, but not vaporize it orotherwise cause it to become physically detached is lost. On the otherhand, continuous wave irradiation at low or moderate intensities isaccumulated in a given area while the beam scans the vicinity of thatarea. Thus at low intensities continuous wave mode may be preferred.Pulsed mode may be the preferred mode at high intensities because if acloud of radiation absorbing material were formed, there would be timefor it to dissipate in the time interval between pulses and, thus, itwould permit a more efficient delivery of radiation to the solidsurface.

Typically, the flexographic printing element is mounted onto an exteriorof a rotating drum associated with the laser. The laser is focused toimpinge on the element on the drum. As the drum is rotated andtranslated relative to the laser beam, the element is exposed to thelaser beam in a spiral fashion. The laser beam is modulated with imagedata, resulting in a two dimensional image with relief engraved into theelement, that is, a three-dimensional element. Relief depth is thedifference between the thickness of the floor and the thickness of theprinting layer. Alternately, the laser may move relative to the elementon the drum.

The laser engravable flexographic printing elements described herein canbe optionally treated to remove surface tackiness either before or afterlaser engraving. Suitable treatments which have been used to removesurface tackiness of styrene-diene block copolymers include treatmentwith bromine or chlorine solutions, and light finishing, i.e., exposureto radiation sources having a wavelength not longer than 300 nm. Itshould be understood that such treatment does not constitute aphotochemical reinforcement of the elastomeric layer. Some embodimentsof the Norrish type II photoinitiators are also operable at thewavelengths used for light-finishing.

In addition, these elements can be subjected to post-laser engravingtreatments such as overall exposure to actinic radiation. Exposure toactinic radiation is generally intended to complete the chemicalhardening process. This is particularly true for the floor and sidewallsurfaces which are created by laser engraving.

EXAMPLES

GLOSSARY Manufacturer/ Identifier Ingredient Supplier Aerosil R-812SHydrophobic silica Evonik CN307 Acrylated polybutadiene oil with highvinyl Sartomer content CN2304 Hexafunctionalized (hyperbranched) esterSartomer acrylate D1192 Styrene-Butadiene-Styrene (SBS) binder Kratonhaving 32% styrene and 8-10% pendant vinyl Polymers groups in themid-block; no di-block content D1119 Styrene-Isoprene-Styrene (SIS)binder Kraton having 22% styrene and 8-10% pendant vinyl Polymers groupsin the mid-block; 65% di-block (SI) Esacure TZT Photoinitiator which isa eutectic mixture of Sartomer 2,4,6-trimethylbenzophenone (CAS 954-16-5) and 4-methylbenzophenone (CAS 134- 84-9) Esacure A122 Tertiary aminefunctionalized with an acrylic Sartomer group (acrylated amine) HiBlackCarbon black Degussa MYLAR ® 601 Polyethylene terephthalate (PET) fillmDuPont Teijin Films SM308 Photoinitiator mixture of a Norrish type IILamberti initiator that is a ketosulphone derivative proprietary toLamberti; a Norrish type I initiator that is Esacure KIP 200 which is 2-hydroxy-2-methyl-1-phenyl-1-propaneone; and a coinitiator that isEsacure A122 which is a tertiary amine functionalized with an acrylicgroup (acrylated amine). The ratio of each component in thephotoinitiator mixture is proprietary to Lamberti. All three componentswere used in the same relative ratio to each other in all examples.SR238 Hexanediol diacrylate Sartomer Vor-Tough A mixture of graphene at10% (by weight) in Vorbeck an SBS binder that is D1192 (from KratonPolymers)

In the following examples, the photopolymerizable layer was formedbetween two film sheets of the MYLAR® 601.

The percent transmittance of ultraviolet radiation through thephotopolymerizable layer (without the PET sheets) was measured on aMcBeth TD904 Densitometer.

Example 1

A mixture of 100 g of D1192, 40 g of D1119, 20 g of CN307, 10 g ofSR238, 10 g of CN2304, SM308, and graphene at a level indicated in thefollowing table) were mixed in a Brabender mixer for 10 minutes at 150°C. The SM308 was at 4% by weight of the composition. The resultingmixture was pressed in a mold in between two polyethylene terephthalate(PET) sheets, to form a photopolymerizable layer 25 mils thick, whichwas cut into three 6″×9″ plate samples.

The transmittance of the plate sample to ultraviolet radiation wasmeasured at about 7-8%.

Example 2

The preparation of the photopolymerizable layer of Example 1 wasrepeated except that the mixture contained graphene at 2% by weight.

The transmittance of the plate sample to ultraviolet radiation wasmeasured at about 2-4%.

Comparative Example 1A and 1B

The preparation of the photopolymerizable layer of Example 1 wasrepeated except that the mixture did not contain graphene.

The transmittance of the plate sample to ultraviolet radiation wasmeasured at about 80-90%.

Comparative Example 2

The preparation of the photopolymerizable layer of Example 1 wasrepeated except that the mixture did not contain graphene, but didcontain Aerosil R-812S at the level indicated in the following table.

The transmittance of the plate sample was measured at about 80-90%.

Example 3

A mixture of 100 g of D1192, 40 g of D1119, 20 g of CN307, 10 g ofSR238, 10 g of CN2304, 13.5 g of Aerosil R-812S, 10 g of Vor-Tough, andSM308 were mixed in the Brabender for 10 minutes at 150° C. The SM308was at 4% by weight of the composition. The resulting mixture waspressed in a mold in between two PET sheets to form a photopolymerizablelayer 25 mils thick, which was cut into three 6″×9″ plate samples.

The transmittance of the plate sample was measured at about 12%.

Example 4

A mixture of 100 g of D1192, 40 g of D1119, 20 g of CN307, 10 g ofSR238, 10 g of CN2304, 11 g of Aerosil R-812S, 20 g of Vor-Tough, andSM308 were mixed in the Brabender for 10 minutes at 150° C. The SM308was at 4% by weight of the composition. The resulting mixture waspressed in a mold in between two PET sheets to form a photopolymerizablelayer 25 mils thick, which was cut into three 6″×9″ plate samples.

The transmittance of the plate sample was measured and was about 3 to5%.

Example 5

A mixture of 100 g of D1192, 40 g of D1119, 20 g of CN307, 10 g ofSR238, 10 g of CN2304, 15 g of Aerosil R-812S, 20 g of Vor-Tough, andSM308 were mixed in the Brabender for 10 minutes at 150° C. The SM308was at 4% by weight of the composition. The resulting mixture waspressed in a mold in between two PET sheets to form a photopolymerizablelayer 25 mils thick, which was cut into three 6″×9″ plate samples.

The transmittance of the plate sample was measured at about 3 to 5%.

Example 6

The preparation of the photopolymerizable layer of Example 4 wasrepeated except that the SM308 photoinitiator in the mixture wasreplaced with Esacure TZT at 3% by weight, and Esacure A122 at 2% byweight. The Esacure TZT is a Norrish type II photoinitiator, and EsacureA122 is a coinitiator.

The transmittance of the plate sample was measured at about 3 to 5%.(Similar to the other examples, the plate samples of Example 6 wereexposed under 365 nm UV light for 5 minutes, and polymerized.)

Example 7

The preparation of the photopolymerizable layer of Example 1 wasrepeated except that the mixture contained graphene at 0.5% by weight.

The transmittance of the plate sample to ultraviolet radiation wasmeasured at about 16-18%.

Control A

The preparation of the photopolymerizable layer of Example 5 wasrepeated except that the SM308 photoinitiator in the mixture wasreplaced with 2-phenyl,2,2-dimethyloxyacetophenone at 4% by weight.

The transmittance of the plate sample was measured at about 3 to 5%. Theplate samples did not polymerize upon exposure to UV radiation, evenafter 20 minutes of exposure.

Control B

A mixture of 100 g of D1192, 40 g of D1119, 20 g of CN307, 10 g SR238,10 a of CN2304, 10 g of HiBlack20, and SM308 were mixed in the Brabenderfor 10 minutes at 150° C. The SM308 was at 4% by weight of thecomposition.

The resulting mixture was pressed in a mold in between two PET sheets toform a photopolymerizable layer 25 mils thick, which was cut into three6″×9″ plate samples.

The transmittance of the plate sample was measured at about 2-3%.

Five dog-bone shaped samples were cut from the plate samples and exposedunder 365 nm UV light for 5 minutes. The PET film was removed from bothsides of the sample. The dog-bone samples were then placed in an Instron3344 (Instron, from Norwood, Mass.) tested for mechanical properties oftoughness and stress-at-break on using a load cell of 100 Newton,according to procedure recommended by the manufacturer. The mechanicaltests on the 5 shaped samples are reported as an average in thefollowing table. Of the remaining plate samples, a piece of 2″×2″ fromeach plate sample was exposed for 5 minutes at 365 nm and then immersedinto a jar containing toluene. The sample pieces were left in thetoluene for 24 hours then dried in air for an hour before theirthickness and the % swelling was measured. The results are presented inthe following table.

Stress- % Dark at- reinforcing % Toughness Break agent Silica (kPa)(kPa) Swelling % Example 1 1% 0 122 108 3.1% graphene Example 2 2% 0 11699 1.6% graphene Comparative 0 0 Not tested Not 100% (sample Example 1Atested not exposed) sample dissolved in solvent Comparative 0 0 123 10614.6% Example 1B (exposed plate) Comparative 0 7 1167 1308 6.1% Example2 Example 3 0.5% 7 1208 1412 4.5% graphene Example 4 1% 5 850 914 2.4%graphene Example 5 1% 7 1285 1533 2.0% graphene Example 6 1% 5 882 9662.3% graphene Example 7 0.5% 0 121 111 4.2% graphene Control A 1% 7 Nottested Not — graphene tested Control B1 5% carbon 0 Not tested Not 100%(sample black tested not exposed) sample dissolved in solvent Control B25% carbon 0 122 110 13.8% black (exposed sample)

Control A versus Example 5, demonstrated the need for aphotopolymerizable layer that contains a dark reinforcing agent to alsoinclude a Norrish type II photoinitiator and coinitiatior.

Controls B1 and B2 demonstrated that the presence of conventional darkreinforcing agent, carbon black, provided no particular advantages inmechanical performance or solvent resistance when compared toComparative Examples 1A and 1B which did not have any reinforcing agent.

Examples 1 and 2 demonstrated that the presence of graphene as a darkreinforcing agent in the photopolymerizable layer surprisingly improvedsolvent resistance of the layer when compared to Control 2B havingcarbon black as the reinforcing agent. Even though Examples 1 and 2 hadminimal improvement of the mechanical properties compared to thosesamples having no reinforcing agents (Comparative B1) or carbon black asreinforcing agent (Control B2), the improvement in solvent resistanceprovides suitable advantage over other photosensitive layers.

Example 6 demonstrated that a Norrish type II photoinitiator (withcoinitiator) is sufficient to cure the photosensitive layer having thedark reinforcing particles of graphene when compared to Example 4 whichincluded a Norrish type I photoinitiator with the Norrish type II andcoinitiator in the photosensitive composition.

Although Comparative Example 2 demonstrated that the presence of silicain the photopolymerizable layer provided some increase the solventresistance (compared to Comparative Example 1B), the addition ofgraphene as shown in Examples 3, 4, and 5 provided significantimprovement in the solvent resistance of the photopolymerizable layer.The lower the % swelling the more resistant the layer is to solventattack. Examples 3 and 5 also demonstrated that the presence of graphenein the photopolymerizable layer provided a significant improvement inmechanical performance compared to Comparative Example 2 which had onlysilica in the photopolymerizable layer. It is expected that the sampleshaving higher values for the toughness and stress-at-break would exhibitimproved wear resistance upon printing on press.

The samples of Examples 4 and 5 were exposed to ultraviolet radiation at365 nm for 10 minutes (1250 Joules/cm²), and imaged using a CYREL®Digital Imager infrared laser radiation exposure unit (CDI Spark 2530made by Esko Graphics Imaging GmbH) at 325 rpm for the drum speed and 18W of power. Typically, the CYREL® Digital Imager is used to selectivelyablate, i.e., remove very thin layer of material but not used forin-depth removal (i.e., in the order of 10-15 micron thickness).However, the present tests were conducted such that the IR laserradiation in the CYREL® Digital Imager engraved the photocured layer,i.e., removed the cured material, in-depth. The plate samples wereengraved with 200 Ipi image having letters approximately 8-12 microns(0.5 mils) deep. It is expected that the samples could be easilyengraved to a depth of 2 to 3 mils if a different laser radiationexposure unit, particularly one with more power, was used.

Example 7 demonstrated a significant improvement in solvent resistancebut had similar performance in mechanical properties when compared toComparative Example 1B. The presence of even 0.5% graphene in the platesample resulted in the sample having solvent swelling of about threefold less than that exhibited by Comparative Example 1B. It is expectedthat even less swelling would be exhibited by the sample of Example 7 insolvents less potent than toluene.

In the above Examples and Comparatives the overall exposure of the platesamples to radiation at 365 nm for 5 minutes was sufficient topolymerize or cure the photopolymerizable layer in the samples, for eventhose plate samples which were dark and contained the Norrish type IIphotoinitiator. It is expected that the photopolymerizable layer of theplate samples would polymerize or cure if exposed to radiation at 354 nmfor about the same time.

1. A photosensitive element for use as a printing form comprising: alayer of a photosensitive composition comprising a binder, a monomer,and a Norrish type II photoinitiator; wherein the photosensitive layercontains reinforcing particles that provide the layer with atransmittance to actinic radiation of less than 20%, and wherein thereinforcing particles are selected from the group consisting ofgraphene, carbon nanotubes, and combinations thereof.
 2. Thephotosensitive element of claim 1 wherein the binder is elastomeric. 3.The photosensitive element of claim 1 wherein the reinforcing particlesare in an amount from about 0.4 to 30% by weight, based on the weight ofthe photosensitive composition.
 4. The photosensitive element of claim 1wherein the photosensitive composition further comprises additionalreinforcing particles selected from the group consisting of carbonblack, graphite, furnace black, dark pigments, and dark toners.
 5. Thephotosensitive element of claim 1 wherein the photosensitive compositionfurther comprises additional reinforcing particles selected from thegroup consisting of TiO₂, calcium carbonate, calcium silicate, bariumsulfate, mica, zinc oxide, magnesium oxide, titanium oxide, aluminum,and alumina, materials having silica-oxygen (Si—O bond) functionality,materials having phosphorus-oxygen (P—O bond) functionality, pigmentparticles, toner particles, and combinations thereof.
 6. Thephotosensitive element of claim 1 wherein the Norrish type IIphotoinitiator is selected from the group consisting of benzophenones,ketosulphones, thioxanthones, 1,2-diketones, anthraquinone, fluorenones,xanthones, acetophenone derivatives, benzoin ethers, benzl ketals,phenylglyoxylates, mono-acylphosphine, and bis-acylphosphine.
 7. Thephotosensitive element of claim 1 wherein the photoinitiator is aketosulphone.
 8. The photosensitive element of claim 1 furthercomprising a coinitiator with the photoinitiator.
 9. The photosensitiveelement of claim 8 wherein the coinitiator is selected from the groupconsisting of thiols, aldehydes, secondary alcohols, primary amines,secondary amines, and tertiary amines.
 10. The photosensitive element ofclaim 8 wherein the photoinitiator is a ketosulphone and the coinitiatoris an amine.
 11. The photosensitive element of claim 1 wherein the layerhas a transmittance to actinic radiation of less than 5%.
 12. Thephotosensitive element of claim 1 wherein the binder includes sufficientpendant alkyl groups of 1 to 6 carbon atoms for donating hydrogen uponreaction with the Norrish type II photoinitiator.
 13. The photosensitiveelement of claim 1 wherein the layer has a thickness from 2 to 250 mils.14. The photosensitive element of claim 1 wherein the photosensitivelayer has a transmittance to actinic radiation of less than 15%.
 15. Thephotosensitive element of claim 1 wherein the photosensitive layer has atransmittance to actinic radiation of less than 10%.
 16. A method forpreparing a printing form from a photosensitive element comprising: a)providing the photosensitive element comprising a layer of aphotosensitive composition comprising an binder, a monomer, and aNorrish type II photoinitiator, wherein the photosensitive layercontains reinforcing particles that provide the layer with atransmittance to actinic radiation of less than 20%, and wherein thereinforcing particles are selected from the group consisting ofgraphene, carbon nanotubes, and combinations thereof; b) exposing thephotosensitive element to actinic radiation; and c) treating the exposedphotosensitive element to form a relief surface suitable for printing.17. The method of claim 16 wherein the exposing step is an imagewiseexposure through a mask to form polymerized portions and unpolymerizedportions, and the treating step is selected from the group consisting of(a) processing with at least one washout solution selected from thegroup consisting of solvent solution, aqueous solution, semi-aqueoussolution, and water; and (b) heating the element to a temperaturesufficient to cause unpolymerized portions to melt, flow, or soften, andremoving the unpolymerized portions.
 18. The method of claim 16 whereinthe exposing step is an overall exposure to cure the layer, and thetreating step engraves the photosensitive element with laser radiationto selectively remove portions of the cured layer.